Accumulation of alpha-synuclein (1-syn) is a shared pathological feature among a group of neurodegenerative diseases collectively referred to as 'synucleinopathies', the most common of which is Parkinson's disease (PD). Lewy bodies (LB) are the histopathological feature of PD and they consist of proteinaceous aggregates with 1-syn as the most prominent component. a-syn gene amplification is responsible for a small subset of familial PD. a-syn overexperssion led to neurodegeneration in a variety of animal models. However, in most sporadic PD, a-syn accumulation is suspected to be a result from insufficient protein degradation. Specific to PD pathogenesis, 1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine (MPTP) has been found to cause Parkinsonism in humans and thereafter widely used as a model for dopaminergic (DA) neuron degeneration in animal models. MPTP inhibits mitochondrial complex I activities and induces a-syn accumulation. a-syn knockout mice are resistant to MPTP toxicity to DA neurons. Transgenic overexpression of a-syn exacerbates MPTP toxicity. How a-syn levels and toxicity are regulated in age-dependent and/or neurotoxin exacerbated PD is a critical issue. High capacity protein degradation pathways include macroautophagy and chaperone-mediated autophagy that transport proteins to the lysosomes. Cathepsin D (CD) is a lysosomal aspartic protease required for completion of autophagy(21). We have shown that CD knockout led to extensive 1-syn accumulation in mouse brains, indicating a role for CD in mediating a-syn metabolism. In vitro we have shown that CD reduces a-syn aggregation and protects against 1-syn-mediated toxicity. To test the hypothesis that CD is neuroprotective against a-syn accumulation and toxicity and to define the mechanism of CD neuroprotection against a-syn accumulation and toxicity, we will perform experiments with the following aims: 1. Test the hypothesis that CD haploinsufficiency leads to an increase of sensitivity to MPTP toxicity in vivo. 2. Test the hypothesis that stereotaxic injection of AAV-CD to the SN attenuates MPTP toxicity in vivo. Completion of these studies will determine the effect of complete and partial loss-of-CD in neurotoxin-induced DA neuron death and a-syn metabolism in vivo, and the effect of gain-of-function of CD in neuroprotection in vivo. Future mechanistic studies will determine how up- or down-regulation of CD affects a-syn metabolism, aggregation and toxicity and whether a-syn toxicity is modulated by mitochondrial dysfunction. These studies will help establish CD as a potential target in PD therapy.